With decade of research and perfection, surface plasmon resonance (SPR) has been widely adopted in the area of chemical and biological sensing. It offers the potential to replace the conventional laborious florescence based biosensing technique. This is because SPR biosensors can provide label-free and real-time quantitative analysis of bio-molecular interactions through monitoring the change in optical response of the functionalized sensing surface usually in terms angular reflectivity, spectral characteristics or corresponding phase shift.
Currently the operation of most SPR biosensors can be classified into three general categories by the measurement scheme of light wave modulated by a surface plasmon: (1) angular reflectivity; (2) spectroscopy; and (3) phase-shift interferometry.
Devices such as Biacore™ T100 or Texas Instruments™ Spreeta, are based on monitoring the position of the minimum in the angle-reflectivity curve when the SPR sensing surface is illuminated by a monochromatic optical beam at a range of incident angles (Enzyme and Microbial Technology, 32, 3-13, 2003). Surface plasmon resonance refers to the effect in which at certain incident angles the p-polarized component of the incident light can couple to a surface plasma wave (SPW) along the interface between a nano-scaled conductive layer on glass prism and the sample medium. This photon-to-plasmon energy transformation is registered as a sharp attenuation of reflectivity and the resonance angle depends on the refractive index of the sample medium. This means that real-time detection of immobilization of biomolecules to a functionalized biosensor surface, which in turn causes a change of refractive index, can be achieved by continuously monitoring the shift of resonance angle. However, the measurement resolution (or Limit of Detection, LOD) of this type of SPR biosensors is only around 10−6 to 10−7 RIU, and this LOD level still does not compare favorably with florescence based techniques for most biosensing applications.
Another approach for SPR biosensing is to adopt spectral measurement. In this configuration, polychromatic light from a halogen lamp is collimated into a large diameter parallel beam, which is directed into a prism coupler. Similar to the angular approach, p-polarized component of the incident light wave is transferred to the SPW, and the transformation is signified as a sharp spectral attenuation dip of the reflected spectrum. To further enhance sensitivity, another possible technique to enhance the detection resolution is achieved by incorporating long-range surface plasmon resonance (LRSPR) excitation in the biosensor thin film stack. When the sensor layer stack is designed in such a way that a dielectric layer sandwiched between two metal layers of appropriate properties, coupled SPWs propagating in both sides of a thin metal film, i.e. LRSPR, may occur. This is a special case of SPR that exhibits a very sharp resonance, hence providing much improved detection resolution. Homola's group proposed to implement the long-range SPR sensor which results improved resolution to 10−8 RIU (Sensors and Actuators B, 123, 10-12, 2007) yet their spectral measurement scheme remained unchanged.
On the other hand, the first practical system for measuring SPR phase was reported by Nelson et al. in 1996 (Sensors and Actuators B, 35-36, 187-191, 1996). The benefit of measuring phase is that the phase change has a steep slope when the system goes through resonance. The rate of change is much higher than the ones due to measuring angular or spectral intensity associated with SPR. This means that theoretically phase measurement may offer better detection resolution.
Ho et al. from the Chinese University of Hong Kong reported a highly sensitive phase-sensitive SPR sensor based on a Mach-Zehnder interferometer and resolution in the order of 10−8 was demonstrated (Optics Letter, 29, 2378-2380, 2004). In this design, a Wollaston prism is placed in the output arm of the interferometer for analyzing the phase change in the p- and s-polarization components. While only the phase change in the p-polarization is associated with SPR, the phase change in the s-polarization is used as the baseline reference. The differential phase between p- and s-polarization components should be free from any common-mode noise, which can be many times larger than the phase signal itself. This also means that one can practically achieve the theoretical resolution limit offered by the phase-sensitive approach. Now recently, the same research group reported that using Michelson interferometer, system sensitivity can be doubled in comparison to single pass Mach-Zehnder device (IEEE Sensors Journal, 7, 70-73, 2007).
Despite that phase-sensitive SPR biosensors provide better detection resolution because of the steep slope across resonance, its measurement dynamic range is known to be narrow in comparison to angle- or spectral-sensitive SPR biosensors. Therefore, achieving wide dynamic range and high sensitivity simultaneously on a single device remains a challenge for all phase-detecting SPR sensors.